1. Field of the Invention
The present invention relates to the field of cellular radio system transmissions. More specifically, the present invention relates to overlapping cellular sectors within such cellular radio systems and ways to improve coverage and capacity in the downlink transmissions of such overlapping cellular sectors.
2. Description of the Prior Art
The rapid growth of telecommunications is perhaps no more acute than in the area of wireless communications. Mobile phones have become a standard personal appliance in nearly every country in the world. In some remote and rural places where the public switched telephone network (PSTN) infrastructure has been slow to advance, wireless communications have even surpassed the PSTN through the use of fixed wireless access technologies. Regardless of whether such wireless communications have been fixed or mobile, they have represented an unprecedented growth in cellular radio system traffic. Such ever-increasing traffic of more and more wireless users has created several problems for network operators including, but not limited to, significant interference among radio signals from the user equipment (UE) and the base stations (BS). Interference impacts upon many aspects of a cellular radio system performance, and it is therefore desirable to minimize such interference.
One basic solution to reduce signal interference between several UE and BS is simply to physically arrange each BS such that transmissions from one do not interfere with the other. Such a solution is not very practical for the reason that placing a BS in such a manner relative to another BS will almost inevitably lead to “dead zones” within which no adequate signal exists. From an interference-avoidance perspective, this is fine. However, any UE within the dead zone will suffer from reduced performance, handoff failures, or outright lack of a carrier signal. Accordingly, most cellular radio systems overlap cell sectors to increase the system coverage. Indeed, beams from the same BS may be designed to overlap each other. This can cause interference and reduce the system capacity significantly.
Other prior art solutions to improve coverage include: (a) soft-handoff (SHO); (b) dynamic interference avoidance; (c) fractional reuse; and (d) tiering. There is a large penalty in capacity because these schemes use additional resources from other cells. In addition, the schemes such as (a) and (b) are dependent on the availability of fast communication between base stations. SHO needs central processing of packets (i.e., frame synchronization for traffic and central MAC) that is difficult to implement under a futuristic distributed architecture. Moreover, per-user throughput distribution becomes better under these schemes, but may not be sufficient.
What is needed therefore is a solution to signal interference within a cellular radio system that is beneficial to both coverage and capacity and that can be implemented under a distributed architecture without requiring dynamic co-ordination among BSs.